Telephone handsets are often used in environments having significant acoustic background noise. When sensed by the telephone microphone, the acoustic background noise signal mixes with a prospective talker's signal, thus reducing the quality and intelligibility of the transmitted talker's signal. Such corrupted talker signals can also present special problems for new digital communications systems where speech coding and compression are utilized.
First-order gradient (FOG) microphones have been used in close-talking applications to discriminate against undesired acoustic background noise. Although, such FOG microphones have been useful in canceling acoustic background noise, an even greater level of acoustic background noise cancellation is desirable.
It is well known that second-order gradient (SOG) microphones can provide an even greater close-talking acoustic noise cancellation than the FOG microphones. SOG microphones often employ two spatially distinct FOG elements, the signals of which are electrically subtracted to obtain the desired second-order spatial differential (or "gradient"). In most prior known arrangements, the spatial sensing points, or "ports", are substantially along a straight line, i.e., they are colinear. Then, the pressure differential may be represented as ##EQU1## where p is the pressure and x is the coordinate along the X axis of the ports. Examples of such SOG microphones are disclosed by G. M. Sessler and J. E. West in an article entitled "Second Order Gradient Uni-Directional Microphone Utilizing An Electret Transducer", JASA, 58 (1975) pages 273-278, and in copending U. S. patent application Ser. No. 08/230,955, filed Apr. 21, 1994, and entitled "Noise-Canceling Differential Microphone Assembly". Alternatively, if the ports are not in a straight line but in a single plane, the pressure differential can be represented as ##EQU2##
For example, see an article by W. A. Beaverson and A. M. Wiggins entitled "A Second-Order Gradient Noise Canceling Microphone Using A Single Diaphragm", JASA, 22 (1950) pages 592-601.
In general, the above SOG microphones achieve enhanced close-talking acoustic background noise cancellation. But, as compared to FOG and omni-directional microphones, they are generally known to have low sensitivity to speech, particularly, at low frequencies where signals from the various ports subtract more completely. This can yield a transmitted talker signal with poor signal-to-electrical noise ratio. Additionally, it has been required to balance the sensitivity and frequency response of the two FOG elements used in the arrangement disclosed in the U.S. patent application Ser. No. 08/230,955, noted above. SOG microphones are also known to be particularly sensitive to the positioning of the handset microphone with respect to the prospective talker's lips, e.g., with distance R from the lips. One solution to this problem is disclosed in U.S. Pat. No. 5,303,307 issued to Elko et al. on Apr. 12, 1994. More recently, the SOG (or more specifically "Broadside SOG") microphone assembly employing colinear ports described in the U.S. patent application Ser. No. 08/230,955, noted above, has made advances in these performance areas. However, further improvement is desirable to insure robust close-talking Broadside SOG microphone performance. A Broadside SOG microphone arrangement employing a single FOG element which eliminates the problems of balancing the FOG elements sensitivity and frequency response is disclosed in copending U.S. patent application Ser. No. 08/237,798, filed May 4, 1994, and entitled "Single Diaphragm Second-Order Differential Microphone Assembly". Another Broadside SOG microphone arrangement is disclosed in copending U.S. patent application Ser. No. 08/333,671, filed Nov. 3, 1994, and entitled "Baffled Microphone Assembly". In these disclosed arrangements the ports are again arranged substantially along a straight line, i.e., are colinear, and the important pressure differential is represented by ##EQU3## Nevertheless, the ports associated with the outer (adjacent) differential ports (or "dipole") are separated by a solid baffle for the purpose of "shielding" the speech waves from the two inner, or the two outer, ports on the rear of the baffle. No arrangement is provided to allow the speech waves to arrive directly at all four ports without shielding via a baffle. This could present problems in certain design applications of their microphone.